# Summary of 3GPP Technical Document

## Document Information
- **Meeting:** RAN2 #131bis (October 2025)
- **Document Number:** R2-2507790 / S4-260015
- **Type:** Reply LS (Liaison Statement)
- **Subject:** RAN simulation assumptions, bundling period and SPS for ULBC
- **Release:** Release 20
- **Work Item:** FS_ULBC (Feasibility Study on Uplink Broadcast)

## Overview
This is a Reply LS from RAN2 to SA4 addressing questions related to protocol overhead, MAC header sizing, and SPS operation for voice transmission in NB-IoT NTN context.

## Main Technical Contributions

### 1. Protocol Overhead for Voice Packet Transmission

#### User Plane (UP) Solution
RAN2 specifies the expected protocol overhead for UP-based voice transmission:
- **PDCP:** 1 byte
- **RLC UM:** 1 byte
- **MAC header:** Variable size (addressed separately)

#### Control Plane (CP) Solution
RAN2 provides two scenarios for CP-based voice transmission:

**Current Specification:**
- **RRC:** 2 bytes
- **RLC AM:** 2 bytes (currently the only specified mode for SRB)
- **MAC header:** Variable size

**Potential Future Enhancement:**
- **RRC:** 2 bytes
- **RLC UM:** 1 byte (if introduced for SRB in case CP solution is selected)
- **MAC header:** Variable size

**Note:** RAN2 indicates that further feedback on expected average RoHC header size may be provided later.

### 2. MAC Header Size Clarification

RAN2 provides detailed guidance on MAC header sizing:
- **Range:** 1 to 3 bytes possible
- **Most common scenario:** 3 bytes total MAC header size is likely
- This clarifies SA4's question about whether 1 byte MAC header is realistic

### 3. SPS Operation with Non-Standard Bundling Periods

RAN2 addresses the feasibility of 120ms and 240ms bundling periods for SPS operation in NB-IoT NTN:

**Key Issue Identified:**
- SPS periodicities of 120ms and 240ms do not divide 10240
- This creates specification challenges

**RAN2 Position:**
- Work on SPS for voice has not yet started in RAN2
- Supporting these non-standard periodicities would require **additional specification work** to resolve the identified issues
- Implies potential complexity and effort required if these periodicities are to be supported

## Actions Requested
RAN2 requests SA4 to take the provided responses into account in their ongoing work on ULBC.